1. Field of the Invention
This invention relates to an apparatus and a method for producing a silicon single crystal by the Czochralski method, and more particularly to an apparatus and a method for producing a silicon single crystal, which achieves uniformity of an oxygen concentration in a pull-up direction of the resultant silicon single crystal.
2. Description of Related Art
It is a general practice in producing a silicon single crystal by the Czochralski method that raw materials put in a crucible, for example, inside a chamber are heated and melted by a heater, and then a seed crystal is dipped in this molten liquid and pulled upwards while it is being rotated, whereby a silicon single crystal grows at the lower end of the seed crystal.
Meanwhile, the following two points are vital to manufacture a semiconductor integrated circuit employing a substrate of silicon single crystal. That is, an adequate amount of oxygen should be contained in the substrate of silicon single crystal so as to gain a so-called Intrinsic Gettering (IG) effect. "Gettering" means a process that removes harmful impurities, e.g. heavy metals, from the regions in a wafer where devices are fabricated. "Wafer" means disk-shaped substrate. IG uses the defects associated with oxygen precipitation in the interior of the wafer, for trapping sites for the harmful impurities. (S.M. Sze, "VLSI Technology" .sctn.1.5.1. McGraw Hill, 1983). Therefore, if the silicon single crystal is employed for a substrate, oxygen of a proper and uniform concentration is necessary. For this purpose, the oxygen concentration in the molten liquid of raw materials in the crucible should be kept constant.
The oxygen which is supplied into the molten liquid from the surface of the quartz crucible through contact therebetween is stirred by a forced convection of the molten liquid due to the rotation of the crucible and heat convection of the molten liquid due to the temperature difference thereof in the crucible. During stirring, the oxygen is not only evaporated from the surface of the liquid in the form of silicon monoxide (SiO), but is carried to the growth surface of the silicon single crystal and taken thereinto. Therefore, when the amount of molten liquid is large in the crucible with a large contact area with the quartz crucible as in an early stage of the crystal growth, the oxygen concentration in the molten liquid is high. On the other hand, as the silicon single crystal proceeds to grow, the amount of the molten liquid in the crucible is decreased and the contact area between the molten liquid and quartz crucible is reduced, whereby the oxygen concentration in the molten liquid becomes lowered. Consequently, the oxygen concentration in the silicon single crystal is generally high in the initial stage of the growth, whereas it is lowered along with the growth of the crystal. The above oxygen concentration in the silicon single crystal is, however, not determined only by the amount of the molten liquid or the contact area with the crucible, but is related also to the dissolved amount of quartz, flow of the molten liquid carrying the dissolved oxygen, evaporation rate of silicon monoxide (SiO), etc. Moreover, the dissolved amount of quartz referred to above is influenced by the temperature of walls of the crucible (H. Hirata and K. Hoshikawa, Jpn. J. Appl. Phys. Vol. 19 No. 8 p. 1573-4 (1980)), namely, the heating distribution of the heater to the crucible. Further, it is also known that the convection of the molten liquid is influenced by the rotation rate of the crucible and single crystal, and the evaporation rate of SiO is influenced by the flow speed of the ambient atmosphere, i.e., Ar gas. These factors are mutually combined each other to determine the oxygen concentration in the silicon single crystal. therefore, it is considerably difficult to maintain constant the oxygen concentration from the start to the end of the crystal growth.
As one method to control the oxygen concentration in the molten liquid, namely, oxygen concentration in the silicon single crystal, Japanese Patent Laid-open Publication Nos. 57-27996 (27996/1982) and 57-135796 (135796/1982) disclose, with noting the relation between the rotation rate of the crucible and the oxygen concentration in the silicon single crystal, changing the rotation rate of the crucible in relation to the amount of the molten liquid, thereby changing the relative speed of the quartz crucible and molten liquid, which results in a forced convection of the molten liquid. Thus, it becomes possible to adjust the thickness of a boundary layer where the oxygen is diffused in the surface of the quartz crucible by the force convection of the molten liquid. Japanese Patent Laid-open Publication No. 62-153191 (153191/1987) discloses another method, wherein the contact area between the quartz crucible and molten liquid and the temperature of the molten liquid are changed. Specifically, according to this latter method, while the supplying ratio of electric power to a plurality of heaters provided in the periphery of side walls of the crucible is adjusted, and accordingly a part of the raw materials are kept in the solid state in the crucible, the silicon single crystal is grown.
According to the former method, however, disadvantages were noticed by the inventors of this invention in that the width of variation of the oxygen concentration cannot be held within .+-.0.5 .times.10 .sup.17 atom /cm.sup.3, or the promotion of the convection of the molten liquid in the crucible develops a large irregularity of a dopant concentration because of the unfixed growing speed. The result of experiments related to the latter method reveals as well that a large shearing stress is undesirably applied to the crucible since the raw materials are repeatedly dissolved and solidified on the crucible, thereby creating a break of the crucible. If these prior art methods are applied to a producing apparatus using a shielding member which has a function to prevent a silicon monoxide deposit from falling into the crucible, a function to prevent a radiant heat from the molten liquid from affecting the pulled-up silicon single crystal and a function to rectify a gas inside the chamber, the high temperature region of the molten liquid is moved upwards in the crucible, thereby reducing the temperature in the lower part of the crucible. As a result, the amount of oxygen dissolved from the bottom of the quartz crucible is reduced, and accordingly the concentration of oxygen taken into the silicon single crystal is reduced. Therefore, a high oxygen concentration cannot be achieved in this producing apparatus through the forced convection of the molten liquid or the like.
In the meantime, a silicon single crystal added with antimony (Sb) as a dopant is also used for a semiconductor substrate. When the Sb doped silicon single crystal is produced by the Czochralski method, since the vapor pressure of diantimony trioxide (Sb.sub.2 O.sub.3) resulting from the addition of Sb is much higher than that of silicon monoxide (SiO), a large amount of oxtgen evaporates from the surface of the molten liquid, causing reduction in the amount of oxygen in the molten liquid. Therefore, as compared with the case when phoshorus, boron or the like is used as a dopant, the oxygen concentration in the produced silicon single crystal becomes considerably lowered. As described earlier, although it may be proposed that the rotation rate of the crucible is rasied to accelerate the convection to increase the amount of oxygen dissolved from the crucible, an interference between the crucible and silicon single crystal which is rotated in a reverse direction to the crucible brings about waves in the molten liquid if the rotation rate of the crucible is raised too much. In consequence to this, the distribution of the dopant in a radial direction becomes not uniform and the silicon single crystal is dislocated. Therefore, the rotation rate of the crucible can be raised only to a certain limit. Even when the Sb doped silicon single crystal of about 0.01 .OMEGA.cm is produced by rotating the crucible at 30 rpm, the oxygen concentration not lower than 13.times.10.sup.17 atom/cm.sup.3 is hardly realized, so that the IG effect can't be expected.
The distribution in a pull-up direction of the dopant taken into the silicon single crystal is determined by the segregation coefficient and initial concentration thereof. The segregation coefficient depends on the convection speed of the molten liquid or rotation rate of the silicon single crystal (J. A. Burton, R. C. Prim and P. Slichter, J. Chem. Phys. 21 p. 1987 (1953)). Since the convection speed depends on the rotation rate of the crucible and single crystal, it is similarly difficult to make uniform the dopant concentration in a radial direction of the silicon single crystal. Accordingly, the specific resistance of the single crystal becomes non-uniform in the radial direction and the yield of the integrated circuit becomes lowered.
In order to solve the above-described problems, such a proposal is made that the molten liquid is positioned in a magnetic field. The convection of the electro-conductive molten liquid is lowered by the applied magnetic field, thereby to stabilize the crystal growth surface, and accordingly the ununiformity in the dopant concentration in the radial direction is improved. However, although the oxygen taken into the silicon single crystal is able to be made uniform in low concentration (not higher than 5.times.10.sup.17 atom/cm.sup.3) through slowing of the convection of the molten liquid in contact with the crucible, a high concentration (not lower than 15.times.10.sup.17 atom/cm .sup.3) of oxygen sufficient to realize the IG effect is hard to be obtained.